1. Field of the Invention
The present invention relates to the field of cell signaling and treatment of cell signaling-related human disorders. More specifically, the present invention discloses a drug target for treatment of hedgehog signaling-related human disorders.
2. Description of the Related Art
The Hh pathway, originally identified in Drosophila (Nusslein-Volhard and Wieschaus, 1980), is important for embryonic development, tissue polarity, cell proliferation and carcinogenesis (Bale, 2002; Pasca di Magliano and Hebrok, 2003; Taipale and Beachy, 2001; Toftgard, 2000; Wetmore, 2003). Inactivation of this pathway causes developmental defects such as holoprosencephaly (Bale, 2002). Activation of this pathway is thought to be responsible for the development of almost all basal cell carcinomas (BCCs) as well as 30% of extracutaneous cancers (Xie, 2005a).
Active Hedgehog proteins are generated from precursor molecules after autoprocessing and lipid modification reactions. Hedgehog signaling proceeds with the binding of Hh ligand to the receptor PTC (Pasca di Magliano and Hebrok, 2003; Taipale and Beachy, 2001). This binding alleviates PTC-mediated suppression of SMO, thus allowing SMO to signal (Pasca di Magliano and Hebrok, 2003; Stone et al., 1996) (FIG. 1). Expression of Hedgehog appears to stabilize the SMO protein (Hooper and Scott, 2005). SMO stabilization triggers formation of a signaling complex in Drosophila, eventually leading to activation of the pathway (Jia et al., 2003; Lum et al., 2003; Ogden et al., 2003; Ruel et al., 2003). However, such a complex does not appear to exist in mammalian cells. SMO ultimately activates transcription factors of the Gli family. Gli molecules enter the nucleus through a nuclear localization signal (Kogerman et al., 1999; Wang and Holmgren, 1999), but little is known about the regulatory mechanism for this process. As transcription factors, Gli molecules can regulate target gene expression by direct association with a consensus binding site (5′-tgggtggtc-3′) located in the promoter region of the target genes (Kinzler and Vogelstein, 1990; Sasaki et al., 1997).
The major breakthrough in the understanding of Hedgehog signaling in human cancers came from the discovery that mutations of PTC homologue 1 (PTCH1) are associated with a rare hereditary form of basal cell carcinoma—Gorlin syndrome (Epstein, 2001; Hahn et al., 1996; Johnson et al., 1996). Most of the mutations in PTCH1 lead to its inactivation, resulting in uncontrolled SMO signaling. Mice that are heterozygous for the PTC-null mutation exhibit a phenotype that resembles Gorlin syndrome (Goodrich et al., 1997; Hahn et al., 1998). Sporadic basal cell carcinomas contain mutations of PTCH1 or SMO (Lam et al., 1999; Reifenberger et al., 2005; Reifenberger et al., 1998; Xie et al., 1998). Mutant SMO, unlike the WT form, is resistant to PTC-mediated inhibition (Murone et al., 1999; Taipale et al., 2000). In basal cell carcinomas, altered hedgehog signaling leads to cell proliferation through elevated expression of PDGFRa (Xie et al., 2001), whereas targeted inhibition of hedgehog signaling causes apoptosis via Fas induction (Athar et al., 2004).
Recent data indicate that Hh signaling is activated in many types of extracutaneous tumors, including medulloblastomas, gastrointestinal, prostate, lung and breast cancers. Unlike the situation in basal cell carcinomas, overexpression of Hedgehog is believed to be responsible for altered hedgehog signaling in most of these tumors (Berman et al., 2003; Watkins et al., 2003). Transgenic mice with pancreatic-specific expression of sonic Hh (Shh) develop pancreatic tumors (Thayer et al., 2003). A subset of esophageal cancers were also found to contain genomic DNA amplification of the Shh gene (Ma et al., 2006a). Activation of the Hh pathway in gastric cancer is associated with tumor progression (Ma et al., 2005; Ma et al., 2006b), and activation of Hh signaling is detected in most metastatic prostate tumors and subsets of locally metastasized tumors (Fan, 2004; Karhadkar et al., 2004; Sanchez, 2004; Sheng et al., 2004). Activation of Hh signaling in small cell lung cancer has also been reported (Watkins et al., 2003), however, a more extensive study indicates that <10% of small cell lung cancers have activated Hh signaling (Chi et al., 2006). Recent studies also suggest that Hh signaling is activated in some breast cancers (Katano, 2005; Liu et al., 2005; Liu et al., 2006).
As a member of the 7-TM protein family, SMO shares homology with the frizzled protein family, with a cysteine-rich N-terminal extracellular domain, 7 transmembrane domains, and a cytoplasmic tail. Several small molecules are identified to associate directly with the TM domain (Chen et al., 2002a; Chen et al., 2002b; Frank-Kamenetsky Kamenetsky et al., 2002). The significance of SMO for Hh signaling in human cancer has been shown by the identification of somatic activating mutations in basal cell carcinomas and medulloblastomas (Lam et al., 1999; Reifenberger et al., 2005; Reifenberger et al., 1998; Xie et al., 1998). In the last few years, intensive studies have been performed to identify the immediate downstream molecule mediating SMO signaling. The presence of Hedgehog results in direct association of SMO with the downstream Costal-2 (Cos-2), causing the transcription factor to remain as an active form (Hooper and Scott, 2005). In contrast, no functional homologues of Cos-2 have been identified in mammalian cells (Varjosalo et al., 2006).
Other studies suggest that SMO signaling may be mediated by other molecules such as b-arrestin 2 and GRK2 (Chen et al., 2004). The unanticipated role of several intraflagellar transport (IFT) proteins in the mammalian Hh pathway indicates a functional connection between the cilia and SMO signaling (Huangfu and Anderson, 2005; Huangfu et al., 2003; Oro, 2006). Gli3 processing is significantly affected by mutations in intraflagellar transport genes (Corbit et al., 2005; Haycraft et al., 2005; Huangfu and Anderson, 2005; May et al., 2005; Olsen, 2005; Zhang et al., 2005). However, we know very little about the ciliary transport of SMO. RanBPM was initially identified as a Ran GTPase binding protein (Nakamura et al., 1998).
Thus, prior art in general, lacks the understanding of the precise involvement of hedgehog signaling pathway in carcinogenesis and other developmental defects. Specifically, prior art is deficient in the knowledge of the molecular mechanism of SMO and molecular interactions downstream of SMO and in the antagonists of the proteins of this pathway that could have substantial therapeutic importance. The present invention fulfills this long-standing need and desire in the art.